Seller backs out – Agent earned commission?

A listing agent produces a ready, willing, and able buyer who enters into a contract with the seller and offers asking price.  The seller backs out of the contract at the last minute because she changed her mind about moving.  The listing agent has:

  1.  Not earned his commission
  2.  Earned his commission
  3.  Earned a portion of his commission
  4.  Earned his commission subject to seller approval

The answer is B –  because the agent has produced a ready, willing, and able buyer, the agent has earned his commission per the terms of the listing agreement.  The agent has a right to ask the seller for his commission.